The present invention relates to a controller of an electric motor, and more particularly to a controller of a synchronous motor suitable for control of stable rotation from a low speed to a high speed.
As a control method of a conventional synchronous motor, a control method described in, for example, Japanese Patent Application No. 62-259399, JP-A-62-114494 and JP-A-1-303091 is described with reference to FIG. 1. When a magnetic pole of a stator is excited in the case where an end (an opposite end in the rotation direction) of a magnetic pole of a rotor is at a position P, the rotor is moved in the direction of arrow and reaches a position R through a position Q. However, since the rotor is continuously excited even in the position R, a braking force acts on the rotor to prevent smooth rotation of the motor.
More particularly, a waveform of an excitation current, as shown by (A) of FIG. 2, is continuously supplied to the stator for some time even after the rotor has reached the position R starting from the position P. Accordingly, since the excitation time is long and a large current flows through the stator as the rotation of the electric motor is low, a consumption power is increased and the rotation of the electric motor is not smooth to be liable to produce vibration.